Purification of the major outer membrane protein of Azospirillum brasilense, its affinity to plant roots, and its involvement in cell aggregation

The major outer membrane protein (MOMP) of the nitrogen-fixing rhizobacteri um Azospirillum brasilense strain Cd was purified and isolated by gel filtr ation, and antiserum against this protein was obtained. A screening of the binding of outer membrane proteins (OMPs) of A. brasilense to membrane-immo bilized root extracts of various plant species revealed different affinitie s for the MOMP, with a stronger adhesion to extracts of cereals in comparis on with legumes and tomatoes. Moreover, this protein was shown to bind to r oots of different cereal seedlings in an in vitro adhesion assay. Incubatio n of A. brasilense cells with MOMP-antiserum led to fast agglutination, ind icating that the MOMP is a surface-exposed protein. Cells incubated with Fa b fragments obtained from purified MOMP-antiserum immunoglobulin G exhibite d significant inhibition of bacterial aggregation as compared with controls . Bacteria preincubated with Fab fragments showed weaker adhesion to corn r oots in comparison to controls without Fab fragments. These findings sugges t that the A. brasilense MOMP acts as an adhesin involved in root adsorptio n and cell aggregation of this bacterium.